30-08-2014, 02:58 PM
ZIGBEE IN INDUSTRIAL AUTOMATION NETWORKS
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ABSTRACT
The promise of ZigBee is now starting to be realized in industrial automation networks; as with any other new technology, ZigBee users must be sufficiently educated in the technology to be able to deploy it successfully. Much of what has been written and said about ZigBee has been focused around home automation, especially lighting control and building automation. However, what is needed for a residential deployment is very different from what is required for an industrial deployment. This paper provides an overview of ZigBee from the perspective of industrial automation including when ZigBee is appropriate and when ZigBee is not appropriate. The paper then discusses the components needed to add ZigBee networks to existing industrial automation networks. Special attention is given to the topic of gateways and options for integrating the ZigBee network with existing industrial automation application software. A methodology is also presented that can be used to design ZigBee networks for situations commonly found on the factory floor. The paper concludes with an overview of the near term advancements expected in the marketplace in relation to ZigBee
INTRODUCTION
Companies are increasingly relying on wireless networks in applications that monitor factory floor conditions and control automated systems. The move to wireless is fueled by the fact that wiring runs are expensive, in terms of both initial runs and reconfiguration, as well as by ongoing performance improvements and cost reductions in industrial wireless technology. ZigBee, a new low-cost, standards-based, wireless mesh networking technology created by the ZigBee Alliance, holds tremendous promise for advancing wireless further in industrial settings, and products supporting the ZigBee standard are just beginning to enter the market. This paper examines the unique needs of wireless industrial automation, the ZigBee components that come together to address those needs and their interoperation, and the promise ZigBee holds for industrial automation as it gains a foothold as a mainstream technology
WHY ZIGBEE IN INDUSTRIAL AUTOMATION?
ZigBee is designed to support easily deployed, redundant path, low-cost systems for integrating even the simplest end devices into wireless automation. While its potential usefulness extends to a wide range of applications that include home as well as industrial, it offers three inherent characteristics that are especially desirable in industrial settings
Low cost
The typical ZigBee radio costs far less than most sensors and signal conditioners, adding about $30 to the cost of any device incorporating one. This provides an economic justification for automating even the simplest of sensors, extending wireless networking’s reach throughout the factory floor
Mesh networking
Traditional wireless networks are point-to-multipoint, with a programmable logic controller (PLC) communicating directly with various devices on the factory floor. Challenges arise when there is a barrier in the line of sight between a PLC and a given end device. ZigBee introduces a mesh networking technology, in which some or all of the end devices with which a PLC communicates also function as routers that can communicate with each other.
MONITORING VS. CONTROL
It’s important in discussing industrial automation to make a distinction between control applications and monitoring applications. Monitoring applications are those that report such conditions as fluid level, pressure, and temperature for the purposes of recording and of issuing alarms when a condition has exceeded a specific acceptable level. Control applications are those that direct automation devices to take action, which can be as simple as opening or closing a valve or as complex as performing real- The distinction between monitoring and controlling is worth noting because ZigBee, as a mesh networking technology, introduces latency when communication is handed off from one routing device to another.
INDUSTRIAL AUTOMATION COMPONENTS:
The typical industrial automation system consists of one or more Programmable Logic Controllers(PLC) plus sensors and actuators working in concert to monitor and/or control a discrete or continuous process
The PLC is the brain of an automation system, programmed to orchestrate network activities. Controllers run ladder logic programs and communicate with end devices to receive information or to instruct them to perform an action. PLCs receive information from sensors, which are end devices that sense and report such conditions as temperature, sound, pressure, strain, and vibration. Sensors are the primary end devices in monitoring applications and provide the inputs upon which control decisions are based
ZIGBEE WIRELESS INDUSTRIAL AUTOMATION
In a ZigBee industrial automation network, ZigBee devices provide an over-the air network interface between the PLC and the sensors and actuators with which it communicates. ZigBee sensor modems provide a combination multichannel signal conditioner and ZigBee module for sensors. Individual versions of ZigBee sensor modems can support various input types, such as 20mA and full-wave bridge, and can provide 4-20mA transmitters, digital I/O, and excitation voltages.
ZigBee Modbus modems connect to actuators, and combine the function of a serial output device with a ZigBee module. Modbus commands issued by the PLC are transported over the ZigBee network and relayed to the I/O module.
ZigBee Modbus gateways provide the takeout point for the ZigBee network, mapping Modbus addresses to ZigBee addresses and formatting responses from ZigBee remote devices into the Modbus protocol. These mappings prevent the Master PLC from having to be ZigBee-aware, and preserve the functionality of existing PLCs, sensors, actuators, and monitoring programs. The application functions exactly as before, except that wireless links have replaced wired connections
ZigBee Modbus address assignments
The mapping of Modbus commands to ZigBee addresses at the ZigBee Modbus gateway is a straightforward process. Modbus assigns a single address to each device in the network, as does ZigBee. The device can be an I/O block that has many sensors and actuator, or an individual sensor or actuator. The only operation required is to convert that address from Modbus conventions to ZigBee conventions, which is performed in the Modbus gateway’s controller software.
ZIGBEE MESH NETWORKING
ZigBee Modbus modems and sensor modems can operate as routers (while still providing I/O), capable of communicating with other routers, gateways, or end devices. The presence of routers is what makes a ZigBee network self-healing. Should a router or end device become unable to communicate directly with the coordinator or primary router due to a dropped link, it will automatically establish an RF link with another router and transmit its data back to the gateway through this second router
ZIGBEE WIRELESS DISTRIBUTED CONTROL NETWORKING
A second common industrial automation architecture is Distributed Control System (DCS), in which multiple PLCs provide local process control. This provides easy segmentation of control functions, especially when there is infrequent interaction between process functions. There is still the need for central monitoring if not supervisory control of the system. The figure below shows data flow between DCS
INDUSTRIAL AUTOMATION – ZIGBEE FUTURE
As ZigBee is a very new standard, the most useful ZigBee products available today are designed to introduce ZigBee benefits to existing networks. The ZigBee gateways, sensor modems, and Modbus modems discussed so far do exactly that, preserving existing investments while replacing expensive and cumbersome wiring with low-cost and flexible wireless connections.
As ZigBee becomes a mainstream technology, we will see the introduction of off-the-shelf ZigBee enabled sensors, actuators, and PLCs. Rather than needing modems attached to these devices, they will contain ZigBee radios themselves to enable cost-effective, ready-to-use devices for creating green field ZigBee networks.
As this happens, ZigBee gateways will continue to be key components, providing interfaces to get ZigBee data off the ZigBee network and onto wired networks that host human-machine interface applications and other applications that make use of the ZigBee data. However, the function of these gateways will change as applications become ZigBee aware, streamlining the mapping process
CONCLUSIONS
As a very well-designed standard created with specific applications in mind, ZigBee has much to offer industrial automation applications. Low-cost deployment and redeployment, mesh networking to cover entire industrial plants and factories, and an open standard with multiple vendors make ZigBee especially adept at addressing the needs of industrial automation applications. As the industrial communications equipment industry continues its embrace of ZigBee, we will see ZigBee devices optimized to meet the unique needs of industrial automation. This will mean products that function well in settings with high RF noise floor, extreme temperatures, and rough handling.
Yet even as the industry evolves toward this optimization, there will for many years be a need for ZigBee products that address existing industrial automation networks and existing applications, and the current crop of solutions – ZigBee modules, sensor modems, and gateways – will likely be keys to implementing ZigBee networks for some time to come.